While reading this sentence, hum your favorite pop tune while writing down the first 15 prime numbers, in order.
Those are three tasks, easy to do separately, basically impossible to do at the same time. If you try, you'll just end up slicing each one into little bits and alternating, almost certainly decreasing the speed and quality of work of each.
Cognitive load slows us down, distracts us and diminishes the quality of the work we do.
We can certainly handle some distraction, in fact, in many cases, a little distraction actually makes things better. Going for a walk, for example, can prompt better ideation than sitting in a dark, silent room might.
The key question for anyone designing software, highways or educational settings is whether or not they are choosing to add productive distraction to our cognitive load.
And for those that seek to be productive, realize that you have a choice about what tools and inputs you're willing to adopt or be distracted by. It's up to you.